152 
PRACTICAL PHOTO-MICROGRAPHY. 
however, is probably not balanced by the extra trouble 
to most workers. But if we have to make a slide from 
a negative larger than three inches we must reduce it. 
This is effected in a camera similar to our enlarging 
one, but turned the opposite way as to the light; the 
larger plate is turned towards the light. The small 
camera contains a slide-plate, and the negative is simply 
copied on a smaller scale qn the slide-plate. 
Again our low-power camera (pages 32, 147) will be 
found very useful for this operation, the slide-plate 
being held in a “ carrier ” in the dark slide. 
Slide-making on Gelatino-chloride Plates. 
These plates ought to be of the slow kind; some 
chloride plates are sold as “ rapid,” being intended for 
use in the camera, but as a rule they are distinctly 
inferior for our purpose to slow chloride plates. These 
plates are only slightly sensitive to yellow light, such as 
gas or candle, and the exposure should be made to day¬ 
light or magnesium ribbon. Two or three seconds to 
daylight diffused by ground-glass will be found suitable 
for an average negative; or two or three inches of 
magnesium ribbon at a distance of a foot may be more 
convenient and equable. The proper exposure must be 
learned by experiment; once known for a few negatives 
it will be easy to judge it for others. The ferrous 
oxalate developer given on page 140 will answer very 
well for these plates ; but a small dose of common salt 
—say one grain to each ounce of developer—may be 
used as a restrainer. The following developer was 
given some years ago by Mr. A. Cowan. The exposure 
